Abstract: Based on the analysis of the monitoring data of Luoshijiang River, Mijuhe River, and Yong’anjiang River, and the northern lake area of Erhai Lake from 2016 to 2018, this paper explored the pollution characteristics of the rivers entering the northern Erhai Lake and its impact on the northern lake area. The results showed that: 1) TN, TP, COD, and ammonia nitrogen in the monitoring section of North Three Rivers fluctuated between wate quality ranks Ⅱ and Ⅲ. The change in pollutant content during the year showed typical agricultural non-point source pollution characteristics, and was affected by the industrial type in the river basin. 2) During the study period, the primary pollutant in the river was TN, the secondary pollutant was COD in the dry season, and TP in the rainy season. The load into the lake in the rainy season was higher than that in the dry season, and the pollution load on Mijuhe River was greater than that of Luoshijiang River and Yong’anjiang River. The pollution load on North Three Rivers into the lake accounted for 50.1% TN and 59.7% TP of the allowable load in Erhai Lake respectively; 3) The input of phosphorus from rivers entering the lake was an important source of phosphorus in the northern Erhai Lake. The correlation between pollutant concentrations in the northern lake area and the pollution load of nitrogen and phosphorus into the lake was stronger than that in the next month, so the response was delayed. Selecting eco-friendly planting models and controlling the load of nitrogen and phosphorus into the lake from the North Three Rivers was beneficial to protect the water environment.